It has been recently proposed within the framework of the 3GPP forum to enhance the Universal Mobile Telecommunication System (UMTS) with a High Speed Downlink Packet Access (HSDPA). The new functionality is aimed at enabling a fast access to packet services through a new transport channel called HS-DSCH for High Speed-Downlink Shared Channel. The physical channels to which the HS-DSCH is mapped, also called HS-PDSCHs (for High Speed Physical Downlink Shared Channels) can be shared by users in the time domain as well as in the code domain. According to his needs, a user is allocated on or more channeling codes (or spreading codes) within the HS-DSCH. Furthermore, the HS-DSCH channel is expected to support Hybrid ARQ and to be capable of accomodating different rates and channel conditions by using Adaptive Modulation and Coding (AMC) schemes.
FIG. 1 represents schematically the channels involved in an HSDPA access. There are basically provided fifteen HS-PDSCH channels intended for the transmission of data from the base station (Node B) to the different user equipments (UEs), four channels called HS-SCCHs (for High Speed Shared Control CHannels) carrying the associated downlink signalling and one uplink channel called HS-DPCCH (for High Speed Dedicated Physical Control CHannel) capable of carrying a feedback information to the base station. The data sub-frames for the differents UEs can be code- and time-multiplexed over the HS-PDSCH channels. More specifically, in a given TTI (Transmission Time Interval) the HS-PDSCH channels can simultaneously accomodate one user to fifteen users. A more complete description of the shared channels in HSDPA can be found in the 3GPP specification TR 25.858 v.5.0.0 which is hereby incorporated by reference.
FIG. 2 shows the process of receiving data from a base station over an HSDPA access. The above mentioned HS-SCCH channels numbered 0 to 3 have been represented in the upper part of the figure while the HS-PDSCH channels numbered 0 to 14 and the HS-DPCCH channel have been represented below. All the channels are subjected to an identical time division into so-called TTIs (for Time Transmission Interval), each TTI corresponding to 3 timeslots in UMTS-FDD (UMTS Frequency Division Duplex). On the other hand, a radio frame is divided into five subframes, a subframe being transmitted during one TTI. A subframe carried by an HS-SCCH channel is called a signalling subframe and a subframe carried by an HS-PDSCH channel is called a data subframe. For example reference numbers 210 to 260 indicate signalling subframes and 211 to 261 indicate data subframes.
The UE monitors the HS-SCCHs channels and checks with its UE ID (UE IDentifier) whether a signalling subframe is assigned to the user. In the affirmative, the UE determines from the content of the signalling subframe the HS-PDSCH channel (among channels HS-PDSCH#0 to HS-PDSCH#14) which carries the data subframe intended to the user. For example, signalling subframe 220 indicates that HS-PDSCH#1 carries a data subframe 221 for the user in question.
The UE checks whether the user data contained in the data subframe are erroneous. If they are not, an acknowledgement indication ACK is transmitted over the HS-DPCCH to the base station, else a negative acknowledgement NACK is sent back in the same way and the base station then retransmits the user data at a later time.
In order to enable the base station to adapt the coding rate and/or the modulation type/order to the transmission conditions, each UE reports at regular intervals the measurement of a parameter representative of the quality of the transmission channel, denoted CQI for Channel Quality Indicator, hereafter referred to as CQI information. For example, if the quality of the transmission is poor, the base station may increase the transmission power, choose a lower coding rate and/or a lower modulation order and, conversely, if the quality of the transmission is high, the base station may choose a higher coding rate and/or a higher modulation order. In practice, the CQI information is coded as a binary word and transmitted to the base station over the HS-PDSCH channel.
FIG. 3 represents schematically the frame structure of the HS-DPCCH channel. Such a frame has a total duration Tf=10 ms and is divided into five TTIs like TTIs 310 to 350, each TTI consisting of 3 timeslots (of duration Ts). A subframe is carried in one TTI and may contain an ACK/NACK information and/or a CQI information. It should be noted that the ACK/NACK information and the CQI information are transmitted independently from each other and that a subframe may contain both information, none of them, or only one of them. More precisely, the ACK/NACK information is transmitted each time the UE receives a data subframe from the base station over the HSPDA access while a CQI information is transmitted at scheduled, periodically distributed transmission times, the determination of which is described into details in the 3GPP specification TS 25.214 v.5.1.0, paragraph 7. The reporting period Tr separating two consecutive reporting times is signalled to the UE by a higher protocol layer and may take different values which are expressed as a number of subframes, namely 1, 5, 10, 20, 40 or 80 subframes.
FIG. 4 schematically shows a time chart of the reporting of a CQI information. The UE estimated the quality of the transmission channel at times t0, t1, t2, etc. and reports these estimates as a CQI information at times t′0, t′1, t′2 distributed at regular intervals Tr. The CQI is typically measured from received pilot symbols transmitted by the base station over the primary CPICH (Common PIlot CHannel) or over a secondary CPICH if beamforming is used. The transmission of the pilot symbols is indicated in the Fig. by the dotted arrows 410, 420, 430. After the channel quality has been estimated, the UE transmits the corresponding CQI information at the next scheduled reporting time. The transmission of the CQI information over the HS-DPCCH channel is indicated in the Fig. by the arrows 411, 421, 431.
The periodic transmission of the CQI allows the base station to track the variations of quality of the transmission channel and to adapt the transmission parameters (e.g. the transmission power, the coding rate or the modulation order) accordingly. However, the quality of the transmission channel may vary very quickly, especially if the UE is moving at high speed or if a corner effect occurs. If the quality drops between two scheduled CQI reporting times, the base station will be unable to modify the transmission parameters to cope with the poor transmission conditions and the packet error rate will increase in consequence. In order to mitigate this effect, a possible measure would be to adopt a shorter reporting period, at the expense however of a significant increase of the level of interference on the uplink because of the more frequent transmission of the CQI information. Conversely, a similar situation occurs when the quality of the transmission channel improves between two scheduled CQI reporting times: in such a situation, the base station cannot profit from the newly improved transmission conditions until the next report has been received.